Communication between the surface and a downhole tool is often conducted during drilling or other oil and gas operations via mud pulse. When transmitting pressure signals from downhole to the surface or from the surface to a downhole transducer, the mud channel causes distortion and attenuation which can impact signal quality. For example, frequency selective fading, where nulls appear in certain parts of the spectrum, can occur because of reflections of signals along the propagation path, the equipment at the surface, changes in characteristics of an channel through the pipe, and/or from the various elements in the bottom hole assembly. Additionally, there is an attenuation that occurs as a function of frequency (higher frequencies are attenuated more) due to the characteristics of the mud in the column.
A system designed to permit communications through this mud channel might adapt in two ways. First such a system may select optimal frequencies in which to operate based on an understanding of the nulls that appear in the spectrum from the channel. In this way, the center frequency of a passband modulation (such as QPSK (Quadrature Phase Shift Keying), BPSK (Binary Phase Shift Keying), MSK (Minimum Shift Keying), SOQPSK (Shaped Offset Quadrature Shift Keying), CPM (Continuous Phase Modulation), QAM (Quadrature Amplitude Modulation), or others) may be adjusted so that no nulls occur in the signal's frequency range, and so that the center frequency is not too high considering the mud column attenuation effects. This can be considered a channel mapping function. Second, equalizers can be employed to reduce the distortion effects of frequency selective fading. Often when equalizers are used, methods are employed to rapidly initiate the equalizers so that they can rapidly converge to a state where the distortion in the channel is largely mitigated.